Vicarious Humanity: By His Birth We Are Healed
This is sometimes referred to as Christ's "vicarious humanity," and is often associated with Scottish theologians like John McLeod Campbell in the 19th century and Thomas and James Torrance in the 20th. But as has already been indicated, this doctrine can be found much earlier in the life of the church, especially in the writings of the Fathers.
With the concept of Christ's vicarious humanity comes a related notion: The characteristics of human life become the property of God the Son, while the characteristics of his divine life become the property of his human nature. In other words, God the Son acquires the qualities of a human being in addition to his divine qualities. He is no longer just a divine person. Now he is a divine person with a human nature. His divinity and humanity are not fused together in some mixture of divine and human characteristics, like many heroes of myth and legend. Rather, his two natures are held together in this one complex person.
An admittedly crude illustration comes from cooking. When we add yeast to dough, the yeast and the dough interact and enable the dough to rise. In one sense, the yeast has become doughy, and the dough has become yeasty—they have taken on the properties of one another.
Of course, the illustration is limited. One difference is this: the dough and yeast get lost in one another, so that one soon forgets the parts and only thinks of something new called "bread." But Christ's human attributes—born in Bethlehem to Mary, raised in first-century Jewish culture in the Middle East—do not compromise his divine nature. Nor does he become something new or different in taking on human flesh. His divinity and his humanity remain intact, yet united in one person. So when he walks and talks and preaches and heals the sick, he is acting as a divine-human being, the God-man.
Theologians like John Calvin describe a wonderful exchange between the qualities of the two natures of Christ—one that jeopardizes the integrity of neither. In the Institutes of the Christian Religion, Calvin puts it like this:
This is the wonderful exchange which, out of his measureless benevolence, he has made with us; that, becoming Son of Man with us, he has made us sons of God with him; that, by his descent to earth, he has prepared an ascent to heaven for us; that, by taking on our mortality, he has conferred his immortality upon us; that, accepting our weakness, he has strengthened us by his power; that, receiving our poverty upon himself, he has transferred his wealth to us; that, taking the weight of our iniquity upon himself (which oppressed us), he has clothed us with his righteousness. (Institutes, 4.17.2)
Several centuries after Calvin, Edwards termed these apparent paradoxes the "admirable conjunction of diverse excellencies." In a sermon titled "The Excellency of Christ," Edwards argues that there is nothing intellectually untoward about ascribing a multitude of differing attributes to Christ. In reality, says Edwards, any misapprehension we might have comes not from any actual incongruities within Christ's nature, but from the inherent limitations of human understanding.
But if Edwards is correct to see in Christ an "admirable conjunction of diverse excellencies," a difficult question arises: Does this mean that his humanity adds to, or improves upon, this package of divine excellence? Does the Incarnation, beyond bringing about our participation in the life of God, add something to God's life as well? Does this "yeast" finally find fulfilment in the dough of humanity?